In the metal casting industry, it is customary to employ vessels such as furnaces, tundishes and ladles to manufacture and transfer molten metals. These vessels are protected against molten metals and slags by expendable refractory linings.
The expendable refractory linings are exposed to severe operating environments due to corrosive attack by the molten slags and metals. The refractory linings also are exposed to thermal shock which can cause premature failure of the refractory.
The severe operating conditions faced by expendable linings reduces substantially the thickness of the linings, thereby precipitating expensive replacement of the linings. The art has attempted to address this problem by providing zoned linings. Zoned linings comprise differing compositions between the top portion of the ladle which contacts molten slag and the lower portion which only contacts the metal. The zoned linings of the art may include, for example, 80-90% alumina brick in the upper portion, and 70% alumina brick in the lower portion. Other types of refractory compositions which have been used for the upper portion of the lining have included magnesia, magnesia-chrome, dolomite, and magnesia carbon.
The art also has attempted to maintain expendable linings by gunning a patch of refractory material onto the damaged portion of the lining. In gunning, a wet mixture having about 7 to 15 percent moisture content is applied to a damaged portion of the lining. Gunning typically is performed after 1-3 inches of lining have been eroded and/or corroded from the original thickness of the lining.
Although gunning has been useful for maintaining linings, gunning is time consuming and requires application of large amounts of material. Typically, gunning requires about 11-14 minutes to apply a one-ton batch of material to the expendable lining of a commercial scale, steel ladle when using an application rate of about 140-180 lbs/minute. Gunning can also require excessive time for drying of the applied material, i.e., 4-6 hours, and generates excessive waste due to rebound. A further disadvantage of gunning is that it produces a rougher surface which is more prone to attack by the slag, and therefore can cause increased erosion of the applied patch.
The art has attempted to improve the corrosion resistance of expendable linings. See W. F. Caley et al., "Chemical and Mineralogical Examination of a 4Cr.sub.2 O.sub.3 MgO Coating Applied to a Bloating Fireclay Brick," Canadian Metallurgical Quarterly, Vol. 26, No. 3, pp. 259-264, (1987). Caley et al. teaches painting a mixture of MgO--CrO.sub.2 onto a fire clay brick. Although Caley et al. teach painting a coating to improve the corrosion resistance of the brick, Caley et al. do not teach maintaining the brick against repeated exposure to slags and molten metals. A need therefore continues for cost effective methods for maintaining expendable refractory linings.